One of the more important commercial fields for polyurethanes is that of coating applications. Early in the development of polyurethane products, it was recognized that they could be used as lacquers; in fact, it was thought that they might extend the areas in which lacquers were used. Thus, coatings upon such substrates such as metals, plastic, paper, textiles, leather, etc. were made possible.
The resulting films exhibited extraordinary adhesiveness, high gloss, good water and solvent resistance, excellent electrical properties, low gas permeability, and good weathering resistance. Also, they could be prepared in various degrees of flexibility and under a wide range of varying temperature conditions. At present, industrial finishes utilizing polyurethane coatings are important because it is possible to take advantage of the inherent flexibility of the polyurethane along with these desirable characteristics in one high performance coating.
Any polyhydroxyl material can be reacted with a diisocyanate bearing component to produce a polyurethane, and the variety of coatings available from these two components is extensive. These two component systems have varying pot lives and could be sprayed on surfaces and allowed to air dry at ambient temperatures or applied and baked at temperatures up to 150.degree. C. Properties such as flexibility and hardness can be varied by the selection of the polyhydroxyl material.
Typically, hydroxyl terminated polyesters with or without additional hydroxyl groups along the polymer chain, hydroxyl terminated polyethers, copolymers of hydroxyl functional acrylates and methacrylates with acrylate and methacrylate esters and styrene, copolymers of allyl alcohol, and other unsaturated monomers such as styrene and hydroxyl containing alkyd resins all have been used as the polyhydroxyl component of such polyurethane coatings. These hydroxyl terminated polymers and copolymers all are high viscosity resins, however, and they require considerable amounts of solvent to reduce their viscosity to the sprayable value of about 25-30 seconds (number 2 Zahn cup) at room temperature.
One way of reducing the viscosity of these high viscosity resins would be to mix them with hydroxyl terminated polyethers, especially those with low molecular weights. However, these low molecular weight polyethers are not suitable for coating applications because of their extremely poor resistance to water, organic solvents and weathering.
Another way of lowering the viscosity of these coating resins is to use low viscosity, low molecular weight isocyanate compounds to react with the polyols described hereinabove. This approach, however, leads to potentially serious health problems, since the necessary isocyanate compounds are often volatile and their vapors can cause serious respiratory difficulties and skin irritations. The potential hazards described are especially serious in a coating operation which utilizes spraying and baking equipment capable of producing high concentrations of isocyanate vapors in the working environment.
Present coating manufacturers have adopted the practice of employing high molecular weight isocyanate prepolymers or isocyanate adducts of low molecular weight polyols such as trimethylol propane to react with the high molecular weight polyols to achieve sprayable viscosities. These prepolymers and adducts are formulated at low NCO/OH ratios in order to minimize the presence of free isocyanate compounds which have high vapor pressures. Consequently, these isocyanate prepolymers or adducts are relatively safe to use, but like the polyols, they are viscous liquids or solids at room temperature. Therefore, when the state of the art coating polyols are mixed with these isocyanate prepolymers or adducts, there is only, at best, a slight reduction in the viscosity of the coating. Thus, considerable amounts of solvent must be added to these compositions in order to achieve a sprayable viscosity.
Sprayable coatings with solvent contents of less than 40% can be prepared when low viscosity polyols are used as chain extenders of the isocyanate prepolymers or adducts. Thus, glycols such as ethylene glycol, propylene glycol and their low molecular weight polymers and copolymers could be used to produce sprayable, high solids polyurethane coatings. The resulting coatings however, are brittle and water sensitive, and would not be considered as high performance coatings.
The present invention eliminates these problems and achieves high solids polyurethane coatings having a sprayable viscosity which can be used in a variety of applications such as for coating plastic parts, various metals, wood and even building material substrates such as concrete, brick and cinder block.
Surprisingly, it has now been found that a number of diricinoleate diols have very low viscosities and are either compatible with or rendered compatible by the addition of small amounts of a cosolvent to the commonly used high viscosity polyols. These diricinoleate derivatives allow the viscosity of the overall coating to be reduced to the sprayable range while not detracting from the coating properties available from the high viscosity polyols. Moreover, as opposed to other known viscosity modifiers such as ethylene glycol, 1-4 butanediol and the like, the use of high amounts of a diricinoleate derivative does not cause embrittlement of the resultant coating films but in fact results in an improvement in the impact resistance and flexibility of these films. Thus, the use of these diricinoleate derivatives alone or the substitution of these derivatives for a portion of the high viscosity polyols yields sprayable coating compositions having improved mechanical properties, good solvent resistance, and satisfactory weathering properties. Furthermore, high performance, commercially useful, high solids sprayable polyurethane coatings can be formulated which contain less than as about 30% solvent.